In mass spectrometry, radio frequency (“RF”) components may be used. Examples of radio frequency components used in a mass spectrometer include ion guides, mass filters, and ion traps. Such RF components may be implemented using a quadrupole configuration. Quadrupole components such as quadrupole mass filters require a temperature-stable RF voltage applied to the quadrupole. Variation in the RF voltage results in a variation from the desired operation of the quadrupole. For the example of a quadrupole mass filter, variation in the RF voltage results in a variation in mass position or mass resolution which degrades the performance of the mass spectrometer.
To stabilize an RF voltage applied to an RF component an RF voltage detector circuit is used to measure the RF voltage. The RF voltage detector circuit includes a feedback control to the RF voltage source to compensate for detected changes in the RF voltage on the RF component. As such, the stability of the RF voltage on the RF component is dependant on the stability of the RF voltage detector to ensure proper detection of changes in the RF voltage.
Such an RF voltage detector circuit or system includes a capacitor used to detect the RF voltage. The stability of the RF voltage detector depends on the ability to maintain the capacitor at a desired capacitance value. Moreover, changes in capacitance as a result of temperature variations, humidity and contamination problems also reduce stability of an RF voltage detector. For example, ceramic capacitors have a temperature drift specification of approximately 30 parts per million per degree Celsius. For this reason, as the temperature of the RF voltage detector changes, so does the capacitance of the capacitor. And, the change in capacitance affects the stability of the RF voltage detector.
Because of the high degree of stability needed in an RF voltage detector, some approaches use custom capacitor designs. These designs can include capacitor designs using air or vacuum dielectric. Typical designs include using materials such as Invar™ and ceramics with custom machining which increase the cost of such designs. In addition, these capacitors are still susceptible to changes in capacitance because of humidity and contamination. Other problems with custom capacitor designs is that the size of the capacitors are very large as required to handle the voltage standoff requirements. As such, placing these large detectors close to the RF components is problematic. The result of not having the RF voltage detector close enough to the RF components is that stray capacitances can result. These stray capacitances further degrade the stability of the RF voltage detector.